Currently honeycomb seals are used in a number of different locations in gas turbines, for example in stages 2 and 3 of the 7E gas turbine that form a seal against rails on shrouded buckets. Honeycomb is used as an abradable system, whereby the rotating rails on the buckets may incur into them during any transient closures between the buckets and the shroud. The incursion into the honeycomb is a sacrificial form of sealing forming a tight clearance, as the intention is that there be no damage to the bucket rails during the interaction. However, the primary problem with honeycomb, particularly in stage 2 shrouds, is its low resistance to oxidation. The honeycomb material is typically 75Ni-16Cr-4.5Al-3Fe-0.05C-0.01Y-0.5Mn-0.2Si-0.1Zr-0.01B (Haynes 214). The temperature range in this location of the turbine is generally in the range of 1500-1700° F. At ˜1600° F., the oxidation life of a 0.005″ thick honeycomb is less than 20,000 hours.
In order to extend the oxidation life of Haynes 214 honeycomb, wall thickness has to be increased to between 0.008-0.011″. However, increasing the honeycomb wall thickness renders the honeycomb less abradable, thus increasing the risk of bucket rail damage during any transient rubs. Alternatively, use of other honeycomb materials such as oxide dispersion strengthened super alloys (e.g. FeCrAlY's) would create additional difficulties such as high costs and brittle behavior (loss of strength) at lower temperatures.
A need exists for an oxidation protected honeycomb seal system which extends the life of the honeycomb while at the same time providing clearance control to reduce hot gas leakage, decreased heat rate and improved turbine efficiency. The present invention seeks to fill that need.